Fermentation method and apparatus

ABSTRACT

A fermentation method and apparatus wherein fermentation medium is continuously circulated between a high pressure region where air and/or oxygen is absorbed and a low pressure region where carbon dioxide is desorbed. The apparatus comprises two connected compartments. During operation differing aeration of the medium in the two compartments produces differing hydrostatic pressures at the lower ends thereof and causes the medium to circulate. The velocity of circulation can be controlled by varying the supply of air to one compartment.

United States Patent Gibson et al.

[ Nov. 12, 1974 FERMENTATION METHOD AND APPARATUS Inventors: Malcolm Ritchie Gibson; Frank Cornelius Roesler; Stuart Raymond Leslie Smith; Frank Peter Maslen, all of Norton, England Imperial Chemicals Industries, Limited, London, England Filed: July 12, 1971 Appl. N0.: 161,787

Assignee:

Foreign Application Priority Data July 21, 1970 Great Britain 35285/70 US. Cl. 195/109, 195/28 R, 195/49, 195/115, 195/142 Int. Cl C12b 1/14 Field of Search 195/109, 142, 115, 95, 195/127 References Cited UNITED STATES PATENTS 12/1942 Seidel 195/95 3,476,366 11/1969 Brooks et a1 195/142 3,546,071 12/1970 Douros et a1. 195/49 3,630,848 12/1971 Lelrancois 195/109 3,625,834 12/1971 Muller 195/109 3,114,677 12/1963 Stich 195/142 Primary ExaminerA. Louis Monacell Assistant Examiner-Thomas G. Wiseman Attorney, Agent, or FirmCushman, Darby & Cushman [57] ABSTRACT 19 Claims, 8 Drawing Figures 1 FERMENTATION METHOD AND APPARATUS tion leaves solution at a suitable rate. In known processes this is usually achieved by performing the fermentation in a stirred fermenter. The operation of the stirrer causes the break-up of bubbles of gas in the fermenter to form a large surface area and hence facilitates the mass transfer of both oxygen and carbon diox- -ide, i.e., oxygen into solution and carbon dioxide out of solution. The use of a stirrer however requires a very high energy input.

The mass transfer of oxygen into the body of the fermentation mixture may be enhanced by a high hydrostatic pressure in the fermenter, i.e. by using a very tall fermenter. However, whilst a high hydrostatic pressure enhances the rate of transfer of oxygen into solution in the lower parts of such tall fermenters, it hinders the rate of transfer of carbon dioxide out of solution.

According to the present invention we provide a method for the aerobic fermentation of a substrate by microorganisms capable of utilizing the substrate for growth wherein the fermentation medium comprising the substrate and microorganisms is continuously circulated between two regions having different hydrostatic pressures, air and/or oxygen being dissolvedin the fermentation medium in the region of higher hydrostatic pressure and carbon dioxide produced during fermentation leaving the fermentation medium in the region of lower hydrostatic pressure.

Further according to the invention we provide a method for the aerobic fermentation of a substrate by microorganisms capable of utilising the substrate for growth wherein the fermentation is conducted in a vessel having two compartments and means to permit the fermentation medium comprising the substrate and microorganisms to circulate continuously between the compartments, air and/or oxygen being admitted to one compartment at or near the lower end thereof to produce a difference between the hydrostatic pressures at the lower ends of the two compartments sufficient to cause the fermentation medium to circulate continuously between two regions having different hydrostatic pressures, air and/or oxygen being dissolved in the fermentation medium in the region of higher hydrostatic pressure and carbon dioxide produced during fermentation leaving the fermentation medium in the region of lower hydrostatic pressure.

Further according to the invention we provide a fermenter, having two compartments and means to permit a fermentation medium to circulate continuously between the compartments, means being provided, to wards the lower end of one compartment, to admit a gas into the fermenter and to produce, when the fermenter contains a fermentation medium, a difference between the hydrostatic pressures at the lower ends of the two compartments sufficient to cause the fermentation medium to circulate continuously between two regions having different hydrostatic pressures, means being provided for gas to escape from the upper part of the fermenter.

Preferably excess heat is removed from the fermentation medium by passing the fermentation medium continuously through a heat exchanger.

The method of the present invention is particularly suitable for use in processes for producing protein or amino acids by growing microorganisms on carboncontaining substrate materials such as carbohydrates, hydrocarbons or partially oxidised hydrocarbons, for example methanol. Preferably the method is used for continuous fermentation.

In the method of the invention, the fermentation is preferably conducted in a vessel wherein the two compartments are situated side by side and are connected by pipes at their upper and lower ends. However the fermentation may also be conducted in a vessel wherein one compartment surrounds the other. In this case it is perferred that air and/or oxygen is admitted at or near the lower end of the outer compartment.

During the operation of the method of the invention, air is admitted at or near the lower end of one compartment (hereinafter referred to as the riser) and rises through the fermentation medium in that compartment which is thus partially occupied by air. Since air is less dense than the liquid, the pressure (P at the lower end of the riser is less than the pressure (P at the lower end of the other compartment (hereinafter referred to as the downcomer) (which contains less air). Thus as the fermentation medium can circulate between the riser and the downcomer the difference between P, and P causes the medium to flow up the riser to the top of the fermenter whilst a downward flow occurs in the downcomer. The fermentation medium therefore is is constantly circulated between a region of high hydrostatic pressure in the lowerpart of the riser, which facilitates the mass transfer of oxygen into the medium, to a region of low hydrostatic pressure in the upper part of the riser, which facilitates the mass transfer of carbon dioxide out of the medium. The rate of flow of the medium is determined by the difference between P and P which depends upon the operating height (as hereinafter defined) of the fermenter and upon the rate at which air is admitted to the riser. Suitable operating heights for the fermenter depend upon the nature of the fermentation and upon the scale of operation.

It should be understood that throughout this specification we mean by the operating height of the fermenter, that portion of the overall height of the fermenter which is occupied by the fermentation medium when the fermenter is in operation.

In a preferred embodiment of the invention, the two compartments, which are preferably cylindrical, are side by side and are connected by pipes at their upper and lower ends, thus completing the circuit for recirculation. Circulation of the fermentation medium is maintained by the pressure difference across the lower connecting pipe caused by differential aeration in the two compartments. Flow is in the direction of decreasing pressure from the lower end of the riser to the lower end of the downcomer. The fermentation medium is thus caused to flow up the riser co-current to air which is sparged with the riser preferably at one or more places in addition to at least one place towards the base of the riser.

The riser is suitably divided vertically into several sections, preferably two, the cross-sectional area of each section being greater than that of the section immediately above it. In the case of two sections most of the mass transfer of oxygen from the gas to the liquid phase takes place in the lower section.

The larger diameter of the lower section increases the liquid and gas hold-up time in the lower section thus permitting maximum solution of oxygen into the liquid phase to maintain the culture under optimum growth conditions. It is important that this greater holdup occurs in the lower sections as this section is maintained at a greater hydrostatic pressure than the upper sections. and increased hydrostatic pressure enhances the solution rate of oxygen into the culture. At a point above the lower section the diameter of the riser is decreased by insertion of a reducing piece in the riser or by some other method. The upper section of the riser, which is preferably cylindrical, is of smaller diameter than the lower section. In this section the upward velocities of both the gas and the fermentation medium is increased because of the reduced diameter and the fraction of the gas-liquid mixture which is gas is greatly increased. The height of the upper section is determined by the hydrostatic pressure required to maintain requisite oxygen solution rates in the lower section, and by the length of time required substantially to desorbe the metabolic carbon dioxide produced during fermentation. The dissolution of carbon dioxide from the culture fluid is enhanced by low hydrostatic pressure and the desorption rate is at a maximum toward the upper end of the riser. Further quantities of air may be added to the upper section of the riser to provide efficient desorption of metabolic carbon dioxide. The additional air or other diluent gas lowers the partial pressure of the carbon dioxide in the gas already in the riser and thereby increases the driving force for desorption.

By this method sufficient oxygen may be transferred from the air or oxygen enriched air to sustain the culture and sufficient carbon dioxide may be transferred from the culture to the gas stream to prevent poisoning of the microorganism.

From the upper end of the riser the gas-liquid mixture enters a pipe connecting the upper ends of the two compartments. Separation of the gas from the liquid occurs along this pipe. The liquid has a horizontal component of velocity along the pipe and the gas has both a horizontal component imparted by the general flow of the mixture, constrained by the walls of the pipe and a vertical component of velocity imparted by the buoyancy forces of the individual bubbles. From a knowledge of the gas and liquid flow rates at this point a suitable free surface area may be selected for the pipe such that disengagement of the gas from the liquid is substantially complete before the liquid re-enters the top ofthe downcomer which contains no air or less air than the riser. The downcomer conveys the fermenting fluid to the lower part of the riser via a lower connecting pipe.

The diameter of the downcomer should be small such that the hold-up time is small compared with the holdup time of the culture in the riser but not so small that the pressure drop created by the high fluid velocities in the downcomer becomes large compared with the hydrostatic head available for circulating the culture between the two regions. Air or oxygen containing air may be added at one or more points in the downcomer of which preferably one is near the top of the downcomer. Air may only be added at a rate such that the average fraction of the gas-liquid mixture which is gas in the downcomer is less than the average fraction of the mixture which is gas in the riser. This air which is added to the downcomer may be used both to control the liquid circulation velocity by adjusting the difference in hydrostatic pressure between the base of the riser and the base of the downcomer and to enable growth of the microorganism to continue in the downcomer by providing a supply of oxygen to the culture.

In both riser and downcomer, optimum gas-liquid mass transfer occurs with gas bubbles having diameters between 1mm and 4mm. It is preferred that the sparging devices supplying air to the fermenter produce bubbles in this region. Although gas bubble coalescence is not severe in fermenting cultures this phenomenon may be reduced by placing bubble break up devices in the path of the co-current flow of gas and liquid in both the riser and downcomer. These devices may be metal, e.g., steel, or plastic meshes, grids, bars, sieve plates or any obstruction which will cause local turbulence. The bubble breakage is enhanced by high velocities in the riser.

In an alternative, less preferred, embodiment of the invention the fermentation is conducted in a fermenter divided along a major proportion of its operating height (as hereinbefore defined), by a partition into inner and outer compartments, the outer compartment surround ing the inner compartment. The compartments are connected at the upper and lower ends of the partition.

The partition may suitably be positioned in the fermenter in a manner such that its lower end is slightly above the base of the fermenter. More suitably the partition extends through a hole in the base of the fermenter and liquid passes through a pipe connected to the lower end of the downcomer, to a heat exchanger and thence is returned through a pipe to the lower end of the riser. Thus the hydrostatic pressure difference produced by air bubbles in the riser may be used to obviate the need for mixture to be pumped through the heat exchanger.

The riser and the downcomer are preferably circular cross-section. The optimum diameter of the downcomer depends upon the nature of the fermentation process being carried out since this determines the optimum velocity required for the liquid passing through the downcomer and the velocity varies inversely with the area of the downcomer. The minimum tolerable velocity depends upon the length of time the particular microorganisms employed in the fermentation can exist in the substantial absence of oxygen, i.e. if the velocity is too small then the microorganisms will die. The maximum tolerable velocity depends upon the amount of pressure drop which can be tolerated at exit ducts in the partition (to permit passage of fluid between the compartments along the length of the partition), i.e. the greater the velocity of the liquid in the downcomer, the greater the loss of pressure head at a duct.

As air bubbles travel upwards through the riser they expand to form larger bubbles which tend to agglomerate to form large slugs of gas in the upper part of the riser. This causes the homogeniety of bubbly liquid to be destroyed and reduces the rate of mass transfer of gases into and out of solution. This problem may be overcome by recirculating downwardly flowing liquid from the downcomer to the riser at points along'the length, preferably at points along the entire length, of the partition. The exit ducts in the partition through which liquid passes from the downcomer to the riser are preferably positioned at points where, for a given air input rate, the rising bubbles have reached a critical voidage fraction. (The voidage fraction is the proportion of the total volume of fluid at a given height which is gas). When liquid is added at these points from the downcomer the voidage fraction is lowered and the upward velocity of liquid in the riser is increased.

The rate of addition of liquid admitted from the downcomer into the riser determines the degree of lowering of the volume fraction of gas phase in the riser. A small rate of addition will lead to a correspondingly small lowering of the volume fraction and will cause the volume fraction of gas in the riser to reach again the critical value after only a short time. A large rate of addition will cause the volume fraction of gas in the riser to fall sharply giving a reduction in the gas liquid surface area available for mass transfer of oxygen and carbon dioxide.

Preferably a balance is struck between having a large number of openings up the height of the fermenter between the riser and downcomer, each giving a small addition of liquid to the riser, and a small number of openings each giving a large addition to the riser.

Preferably these openings are formed as annuli from two concentric tubes of different diameter, the tube with the larger diameter being situated above the tube with the smaller diameter. The rate of flow of liquid from the downcomer to the riser may be calculated from the difference in the area between the larger tube and the smaller tube, and the linear velocity of the liquid in the downcomer. The advantage of this method is its mechanical simplicity and the minimization of turbulance in the downcomer. The liquid being admitted to the riser may be dispersed intimately into the liquid and gas in the riser by deflector plates which dispose the axial liquid flow from the downcomer wholly or partly in the radial direction.

Alternatively spouts or scoops may be used to direct the downward flow of liquid in the downcomer into the riser. The spouts or scoops are pipes or tubes placed inside the downcomer to catch the liquid; a hole is drilled in the downcomer to admit the spout or scoop into the riser to allow intimate mixing between the liquid jetting out of the scoop and the bubbly liquid in the riser. The scoop or spout may be of any cross-section, preferably circular, and may be bent through any suitable angle to the vertical to promote good mixing. There may be any number of such scoops or spouts arranged around the circumference of the downcomer. Below the level of exit of the scoops or spouts the downcomer may be reduced in diameter to maintain the liquid velocity with the aid of a reducing piece.

To prevent excessive bubble coalescence which would reduce mass transfer rates, strainers, of e.g. plastic or steel mesh may be inserted into the riser to break up the bubbles.

in this embodiment, there is a tendency for some bubbles, particularly smaller ones, to fail to break the surface of the liquid on rising to the top of the riser and to be carried over into the downcomer. To reduce the tendency of bubbles to carry over in this fashion the upper end of the partition may be formed to slope outwards, preferably to form a frustum of a cone at its upper-end. In this manner the free surface area available fo the disengagement of bubbles at the top of the fermenter will be increased. Bubbles will additionally be separated out through the action of the sloping portion of the partition. Bubbles having reached the partition must then travel along the underside of the sloping portion to reach the surface, i.e. must travel at an angle to the vertical whilst their normal motion is vertical, and are afforded more opportunity to break the surface. The size of the sloping portion at the top is determined by the probable size of the bubbles at the surface, i.e. the length of the sloping side preferably varies inversely with the expected size of the bubbles. The slope may be at any angle to the vertical (it may be horizontal) but is preferably at 45 to the vertical. In order to eventually collect bubbles separated by the partition, the sloping portion may be pierced by a number of pipes or chimneys extending above the surface of the liquid in the fermenter, up which large bubbles may pass vertically and escape rapidly. Preferably the chimneys are equally spaced around the upper end of the riser. The chimneys may be of any cross-sectional shape e.g., circular, or they may be formed by semicircular baffles attached to the outside rim of the sloping partition. The area of each chimney depends upon the amount of gas fed into the fermenter and upon the number of chimneys. To avoid the physical carrying over of liquid the chimneys preferably project for some distance above the general liquid level.

There should always be at least one air inlet at or near the lower end of the riser. However, if desired, supplementary air and/or oxygen inlets to the riser may be provided at other points along the length of the riser. If the process is to operate continuously, openings in the wall of the fermenter can be provided at suitable positions for theaddition of fresh reactants and for the removal of products. If the carbon containing substrate is gaseous it may be admitted to the fermenter through openings at or near the lower end of e.g. the riser, either with the air or oxygen or separately.

Fermenters of the present invention can be designed to have the following advantageous features:

1. The use of mechanical stirrers is avoided. The liquid is stirred by the rising bubbles in the riser and overall circulation is maintained to keep the liquid homogeneous by the pressure head developed.

2. The fermenter affords a hydrostatic pressure difference in which oxygen is mainly absorbed at higher pressure and CO is mainly desorbed at lower pressure.

3. The fermentation mixture is delivered rapidly and continuously between these areas of hydrostatic pressure difference thus avoiding exposing the microorganisms to either high 0 or CO liquid phase partial pressures.

4. The flow of the fermentation mixture is driven by hydrostatic forces, which are provided by dividing the fermenter into a riser and a downcomer and admitting considerably more air to the former.

The invention is illustrated by the accompanying drawings wherein:

FIGS. 1 to 6 illustrate two forms of the fermenter of the invention having the riser surrounding the downcomer.

FIG. 7 illustrates a more preferred form of the fermenter with riser and downcomer side by side..

FIG. 8 is a schematic line diagram of a plant for the continuous production of a proteinaceous food supplement employing the method according to the inventron.

FIG. 1 is a cross-sectional elevation of a portion of the operating length of a fermenter according to the invention.

FIG. 2 is a plan view of the top of the fermenter.

FIG. 3 is a cross-section along the line AA of FIG. 1.

FIG. 4 is a side elevation of the top of the fermenter.

FIG. 5 is a cross-sectional elevation of a portion of the operating length of an alternative embodiment of the fermenter.

FIG. 6 is a cross-section along the line AA of FIG. 5.

FIG. 7 is a side elevation, partly in cross-section, of the preferred form of fermenter.

The preferred form of fermenter shown in FIG. 7 has a riser comprising cylindrical upper and lower sections 24 and 25 respectively connected through a reducing piece 34, upper section 24 having a smaller diameter than lower section 25. The upper section 24 of the riser is connected through upper connecting piece 29 to the upper end of cylindrical downcomer 28 whilst lower section 25 is connected through lower connecting piece 26 with the base of downcomer 28. Air is sparged into lower section 25 through sparge pipes 27 causing continuous circulation of fermentation medium which occupies the fermenter up to level c-c. Upper connecting piece 29 is not allowed to run full of liquid to allow a free surface from which air and carbon dioxide escape from the medium and pass out through port 30. Additional air is sparged into the upper part of downcomer 28 through pipe 33. Substrate medium enters the fermenter through pipe 37 whilst product is removed through pipe 35. Additional nutrients such as ammonia for example, may be fed to the fermenter through pipe 36. Upper section 24 of the riser contains a series of bubble break up devices such as 38. Downcomer 28 contains heat exchanger 31.

The fermenter shown in FIGS. 1 to 4 has a cylindrical outer wall 12 enclosing riser l3 and downcomer l4 separated from one another by inner wall 15. Inner wall comprises a series of separate cylindrical sections, section being of smaller diameter than the section immediately above it. At the junction between each pair of sections, wall 15 is pierced by annular exit ducts 16. The sections are held in position relative to each other by struts l7.

Deflector plates 21 below exit ducts l6 enable liquid passing out ofdowncomer 14 to be dispersed intimately into the liquid and gas in riser 13. At any junction between sections, the wall of the upper section may be extended below the junction to form a skirt 20. Skirt may be parallel to inner wall 15 as shown in FIG. 1 or may be flared outwards at an angle, of for example 8, to the wall of the upper section. Junctions at which the wall of the upper section is continued as a flared skirt are most suitable in the upper parts of the fermenter. Downcomer 14 is connected to riser 13 by a pipe (not shown in the drawings) which passes from downcomer 14 through the base (not shown in the drawings) of the fermenter to a heat exchanger (not shown in the drawings) and thence through the base to riser 13. The base is also pierced by a series of sparging devices (not shown in the drawings) through which air or oxygen can be admitted to riser 13. At the upper end of the fermenter, outer and inner walls 12 and 15 slope diagonally outwards, the uppermost section of inner wall 15 terminating in a frustum of a cone 22 through which pass chimneys 23. Outer wall 12 extends for some distance above the upper ends of cone 22 and chimneys 23.

In the alternative, less preferred, embodiment shown in FIGS. 5 and 6, wall 15 is continuous, being tapered at intervals to form sections, each of which is smaller in diameter than the section immediately above. At the junction between each pair of sections, wall 15 is pierced by exit ducts 18, the sides 19 of which project into the riser to form scoops.

During the operation of the method of the invention using a fermenter according to FIGS. 1 to 4, the fermentation mixture occupies the space up to the line BB of FIG. 4. Air is admitted at a suitable rate to riser 13 and bubbles rise upwardly through riser 13. On reaching cone 22, small bubbles which have not quite risen to the surface travel along the underside of 22 and gradually rise towards the surface to escape before being carried over into downcomer 14. Of those few smaller bubbles carried over into the upper part of downcomer 14 some escape from the surface of the liquid in 14 and few are carried down into downcomer l4. Larger bubbles escape through chimneys 23. Since riser 13 contains more bubbles than downcomer l4, pressure P at the base of the riser is less than pressure P at the base of the downcomer. Thus liquid passes upwardly in 13 and downwardly in 14. Portions of the downwardly flowing liquid are recycled to 13 through ducts 16 distributed down the length of partition 15. Liquid passing to the lower end of 14 passes through a pipe to a heat exchanger and is then returned to riser 13.

In the plant shown in FIG. 8 a carbon containing substrate continuously passes along pipe 1 into stirred mixer tank 2 where it is continuously diluted with water, a nitrogen-containing compound and inorganic salts entering through pipe 3 to form an aqueous substrate for the fermentation process. The aqueous substrate formed in the mixer tank passes continuously through a sterilizer (not shown in the drawing) into fermenter 5 into which an inoculant of microorganisms has been introduced. Air under pressure passes into the fermenter through pipe 6 and filter 7.

Ammonia enters the fermenter through pipe 8 and filter 9 whilst additional air is admitted through pipe 4.

In the fermenter the proteinaceous product is formed as a slurry which is continuously passed to centrifuge 10 from whence the solid proteinaceous product is passed to drier 11 whilst unfermented liquid is returned to mixer tank 2. Throughout the process the fermenter is maintained at a temperature of about 30C. In the drier, the proteinaceous product is dried by hot air at a temperature of I00C 300C before being removed through pipe 12.

We claim:

1. A method for the aerobic fermentation of a substrate by microorganisms capable of utilizing the substrate for growth, comprising the steps of continuously circulating fermentation medium comprising the substrate and microorganisms through a system having a compartment of ascending flow and a compartment of descending flow, connected at their upper and lower ends, admitting an oxygen-containing gas to the lower end of the ascending flow compartment to provide a hydrostatic pressure difference at the lower ends of the two compartments so as to cause said medium to circulate between two regions having different hydrostatic pressures, and controlling the velocity of the liquid circulation by sparging oxygen-containing gas to the upper part of the compartment of descending flow, the amount of gas thus admitted to said compartment of descending flow being less than the amount of gas admitted to said compartment of ascending flow, the gas admitted to said system being the sole essential means for stirring and circulating said medium and the dimen sions of the system being such that in the ascending flow compartment the medium is transported continuously and rapidly between a lower region of higher hydrostatic pressure, in which oxygen is absorbed, and an upper region at a lower hydrostatic pressure, in which carbon dioxide produced during fermentation is desorbed, at a rate such that the carbon dioxide liquid phase partial pressure to which the microorganisms are subjected is within the range tolerable to the microorganisms.

2. A method according to claim 1 wherein the crosssectional area of the compartment of ascending flow is greater than that of the compartment of descending flow.

3. A method according to claim 1 wherein the compartment of ascending flow is divided vertically into sections each section being of greater cross-sectional area than the section immediately above it.

4. A method according to claim 3 wherein the compartment of ascending flow is divided into two sections.

5. A method according to claim 1 wherein the two compartments are situated side by side and are connected at their upper and lower ends.

6. A method according to claim 1 wherein the substrate is selected from the group consisting of hydrocarbons and oxygenated hydrocarbons.

7. A method according to claim 6 wherein the oxygenated hydrocarbon is methanol.

8. A method according to claim 1 wherein excess heat is removed from the fermentation medium by passing the fermentation medium continuously through a heat exchanger.

9. A fermenter having two compartments constituting a riser and a downcomer, conduit means to permit fermentation medium to circulate continuously between the compartments, means towards the lower end of the riser to admit a gas into the fermenter and means to allow gas to escape from the upper part of the fermenter, and means for sparging gas into the upper end of the downcomer to control the velocity of the medium circulating through the compartments, the dimensions of the fermenter being such that when gas is admitted towards the lower end of the riser, fermentation medium contained in the fermenter is caused to circulate continuously and rapidly between a lower region of the riser of higher hydrostatic pressure in which gas is absorbed and an upper region of the riser of lower hydrostatic pressure wherein gas is desorbed, at a rate such that the microorganisms in the fermentation medium are subjected to a liquid phase partial pressure of gas produced during the fermentation which is within the range tolerable to the microorganisms.

10. A fermenter according to claim 9 wherein the cross-sectional area of the riser is greater than that of the downcomer.

11. A fermenter according to claim 9 wherein the riser is divided vertically into sections each section being of greater cross-sectional area then the section immediately above it.

12. A fermenter according to claim ll wherein the riser is divided into two sections.

13. A fermenter according to claim 9 wherein the two compartments are situated side by side and are connected at their upper and lower ends.

14. A fermenter according to claim 9 wherein means are provided for admitting gas to the riser at a plurality of points along its length.

15. A fermenter according to claim 1 wherein means are provided in the riser to reduce gas bubble coalescence.

16. A fermenter having two compartments constituting a riser and a downcomer, conduit means to permit fermentation medium to circulate continuously between the compartments, means towards the lower end of the riser to admit a gas into the fermenter and means to allow gas to escape from the upper part of the fermenter, and means for sparging gas into the upper end of the downcomer to control the velocity of the medium circulating through the compartments, the dimensions of the fermenter being such that when gas is admitted towards the lower end of the riser, fermentation medium contained in the fermenter is caused to circulate continuously and rapidly between a lower region of the riser of higher hydrostatic pressure in which gas is absorbed and an upper region of the riser of lower hydrostatic pressure wherein gas is desorbed, at a rate such that the microorganisms in the fermentation medium are subjected to a liquid phase partial pressure of gas produced during the fermentation which is within the range tolerable to the microorganisms wherein means are provided to reduce gas bubble coalescence comprising at least one grid.

17. A fermenter having two compartments constituting a riser and a'downcomer, conduit means to permit fermentation medium to circulate continuously between the compartments, means towards the lower end of the riser to admit a gas into the fermenter and means to allow gas to escape from the upper part of the fermenter, and means for sparging gas into the upper end of the downcomer to control the velocity of the medium circulating through the compartments, the dimensions of the fermenter being such that when gas is admitted towards the lower end of the riser, fermentation medium contained in the fermenter is caused to circulate continuously and rapidly between a lower region of the riser of higher hydrostatic pressure in which gas is absorbed and an upper region of the riser of lower hydrostatic pressure wherein gas is desorbed, at a rate such that the microorganisms in the fermentation medium are subjected to a liquid phase partial pressure of gas produced during the fermentation which is within the range tolerable to the microorganisms divided along a major proportion of its operating height by a partition into inner and outer compartments, the outer compartment surrounding the inner compartment.

18. A fermenter according to claim 17 wherein the outer compartment is the riser.

19. A fermenter according to claim 17 wherein the two compartments are connected at their upper and lower ends and at least one point in between. 

1. A METHOD FOR THE AEROBIC FERMENTATION OF A SUBSTRATE BY MICROORGANISMS CAPABLE OF UTILIZING THE SUBSTRATE FOR GROWTH, COMPRISING THE STEPS OF CONTINUOUSLY CIRCULATING FERMENTATION MEDIUM COMPRISING THE SUBSTRATE AND MICROORGANISMS THROUGH A SYSTEM HAVING A COMPARTMENT OF ASCENDING FLOW AND A COMPARTMENT OF DESCENDING FLOW, CONNECTED AT THEIR UPPER AND LOWER ENDS, ADMITTING AN OXYGEN-CONTAINING GAS TO THE LOWER END OF THE ASCENDING FLOW COMPARTMENT TO PROVIDE A HYDROSTATIC PRESSURE DIFFERENCE AT THE LOWER ENDS OF THE TWO COMPARTMENTS AO AS TO CAUSE SAID MEDIUM TO CIRCULATE BETWEEEN TWO REGIONS HAVING DIFFERENT HYDROSTATIC PRESSURES, AND CONTROLLING THE VELOCITY OF THE LIQUID CIRCULATION BY SPARGING OXYGEN-CONTAINING GAS TO THE UPPER PART OF THE COMPARTMENT OF DESCENDING FLOW, THE AMOUNT OF THE GAS THUS ADMITTING TO SAID COMPARTMENT OF DESCENDING FLOW BEING LESS THAN THE AMOUNT OF GAS ADMITTED TO SAID COMPARTMENT OF ASCENDING FLOW, THE GAS ADMITTED TO SAID SYSTEM BEING THE SOLE MEANS ESSENTIALLY FOR STIRRING AND CIRCULATING SAID MEDIUM AND THE DEMINSION OF THE SYSTEM BEING SUCH THAT IN THE ASCENDING FLOW COMPARTMENT THE MEDIUM IS TRANSPORTED CONTINUOUSLY AND RAPIDLY BETWEEN A LOWER REGION OF HIGHER HYDROSTATIC PRESSURE, IN WHICH OXYGEN IS ABSORDED, AND AN UPPER REGION AT A LOWER HYDROSTATIC PRESSURE, IN WHICH CARBON DIOXIDE PRODUCEDD DURING FERMENTATION IS DESORBED, AT A RATE SUCH THAT THE CARBON DIOXIDE LIQUID PHASE PARTICAL PRESSURE TO WHICH THE MICROORGANISMS ARE SUBJECT IS WITHIN THE RANGE TOLERABLE TO THE MICROOGRANISMS.
 2. A method according to claim 1 wherein the cross-sectional area of the compartment of ascending flow is greater than that of the compartment of descending flow.
 3. A method according to claim 1 wherein the compartment of ascending flow is divided vertically into sections each section being of greater cross-sectional area than the section immediately above it.
 4. A method according to claim 3 wherein the compartment of ascending flow is divided into two sections.
 5. A method according to claim 1 wherein the two compartments are situated side by side and are connected at their upper and lower ends.
 6. A method according to claim 1 wherein the substrate is selected from the group consisting of hydrocarbons and oxygenated hydrocarbons.
 7. A method according to claim 6 wherein the oxygenated hydrocarbon is methanol.
 8. A method according to claim 1 wherein excess heat is removed from the fermentation medium by passing the fermentation medium continuously through a heat exchanger.
 9. A fermenter having two compartments constituting a riser and a downcomer, conduit means to permit fermentation medium to circulate continuously between the compartments, means towards the lower end of the riser to admit a gas into the fermenter and means to allow gas to escape from the upper part of the fermenter, and means for sparging gas into the upper end of the downcomer to control the velocity of the medium circulating through the compartments, the dimensions of the fermenter being such that when gas is admitted towards the lower end of the riser, fermentation medium contained in the fermenter is caused to circulate continuously and rapidly between a lower region of the riser of higher hydrostatic pressure in which gas is absorbed and an upper region of the riser of lower hydrostatic pressure wherein gas is desorbed, at a rate such that the microorganisms in the fermentation medium are subjected to a liquid phase partial pressure of gas produced during the fermentation which is within the range tolerable to the microorganisms.
 10. A fermenter according to claim 9 wherein the cross-sectional area of the riser is greater than that of the downcomer.
 11. A fermenter according to claim 9 wherein the riser is divided vertically into sections each section being of greater cross-sectional area then the section immediately above it.
 12. A fermenter according to claim 11 wherein the riser is divided into two sections.
 13. A fermenter according to claim 9 wherein the two compartments are situated side by side and are connected at their upper and lower ends.
 14. A fermenter according to claim 9 wherein means are provided for admitting gas to the riser at a plurality of points along its length.
 15. A fermenter according to claim 1 wherein means are provided in the riser to reduce gas bubble coalescence.
 16. A fermenter having two compartments constituting a riser and a downcomer, conduit means to permit fermentation medium to circulate continuously between the compartments, means towards the lower end of the riser to admit a gas into the fermenter and means to allow gas to escape from the upper part of the fermenter, and means for sparging gas into the upper end of the downcomer to control the velocity of the medium circulating through the compartments, the dimensions of the fermenter being such that when gas is admitted towards the lower end of the riser, fermentation medium contained in the fermenter is caused to circulate continuously and rapidly between a lower region of the riser of higher hydrostatic pressure in which gas is absorbed and an upper region of the riser of lower hydrostatic pressure wherein gas is desorbed, at a rate such that the microorganisms in the fermentation medium are subjected to a liquid phase partial pressure of gas produced during the fermentation which is within the range tolerable to the microorganisms wherein means are provided to reduce gas bubble coalescence comprising at least one grid.
 17. A fermenter having two compartments constituting a riser and a downcomer, conduit means to permit fermentation medium to circulate continuously between the compartments, means towards the lower end of the riser to admit a gas into the fermenter and means to allow gas to escape from the upper part of the fermenter, and means for sparging gas into the upper end of the downcomer to control the velocity of the medium circulating through the compartments, the dimensions of the fermenter being such that when gas is admitted towards the lower end of the riser, fermentation medium contained in the fermenter is caused to circulate continuously and rapidly between a lower region of the riser of higher hydrostatic pressure in which gas is absorbed and an upper region of the riser of lower hydrostatic pressure wherein gas is desorbed, at a rate such that the microorganisms in the fermentation medium are subjected to a liquid phase partial pressure of gas produced during the fermentation which is within the range tolerable to the microorganisms divided along a major proportion of its operating height by a partition into inner and outer compartments, the outer compartment surrounding the inner compartment.
 18. A fermenter according to claim 17 wherein the outer compartment is the riser.
 19. A fermenter according to claim 17 wherein the two compartments are connected at their upper and lower ends and at least one point in between. 